Optical Detection of Individual Molecules

By shining circularly polarised light on a gold, square-ring shaped nanostructure, the researchers were able to ‘activate’ the entire surface of the nanostructure, thereby significantly increasing the opportunity for interaction with molecules. The method has a broad range of potential applications in nanoscale photochemistry and could assist in the advancement of technologies for visualising single molecules and multiple-molecule interactions.

Nanotechnology researchers around the world are exploring ways to optically detect single molecules, but progress can be hindered by the fact that single molecules have extremely weak optical responses. Thus far, scientists have developed a way to use metal nanostructures to focus light into tiny spots called ‘hotspots’. The hotspots excite electrons on the surface of the nanostructure, causing them to oscillate coherently. When shone on a molecule, and with the help of these oscillating electrons, the focused light can increase a molecule’s optical signal to 100 billion times its normal strength. This signal can then be detected with an optical microscope.

But there are two limitations to the current method: hotspots can become too hot, and they are just spots. That is, the heat from hotspots can melt the nanostructures, thus destroying their ability to channel light effectively, and hotspots produce only a very small cross-section in which interaction with molecules can take place. Additionally, for a single molecule to become detectable, it needs to find the hotspot.

In order to overcome these limitations, Doctor Valev and his colleagues sought out to nanoengineer larger spots. They began by shining circularly polarised light rather than linearly polarised light on the nanostructures and found that this could increase the useful area of these nanostructures. More importantly, when shone on square-ring shaped gold nanostructures, the scientists observed that theentire surface of the nanostructures was successfully activated.

Valev explains: “Essentially, light is constituted of electric and magnetic fields moving through space. While with linearly polarised light, the fields move in a linear, forward direction, with circularly polarised light, they rotate in a spiral-like motion." The circularly polarised light imparts a sense of rotation on the electron density in ring-shaped gold nanostructures, thus trapping the light in the rings and forming ‘loops of light’. The loops of light cause excited electrons to oscillate coherently on the full surface of the square-ringed nanostructures – rather than in a few concentrated hotspots. This increases the opportunity for interaction with molecules: “The trick is to try to activate the whole surface of the nanostructure so that whenever a molecule attaches, we will be able to see it,” says Valev. “That is precisely what we did.”